1. Field of the Invention
The invention relates generally to grit blast cutting of materials.
2. Description of the Prior Art
The best known prior art is depicted in FIG. 1, a detailed showing of an abrasive feeder type C6015-1 produced by Flow Systems, Inc. of Kent, Wash. The abrasive valve assembly A includes a hopper B, for storing the abrasive grit, a hopper needle valve C for metering grit into the system on demand. The needle valve is seated in seat D when no grit delivery is required at the nozzle. When grit delivery to the nozzle is required the needle valve is caused to translate downwardly by admitting air pressure above the needle valve. With the needle valve displaced from the seat grit is delivered to the nozzle. When flow of grit is to be terminated, the air pressure is removed whereby spring bias, not shown, returns the needle valve upwardly into seat D terminating the flow of grit to the nozzle. This cycle is repeated as grit is supplied to and terminated from the nozzle. The problem with this grit delivery system is that the grit, although generally uniformly sized, includes grit particles of a size larger than the selected grit size and these larger grit particles hang up between the needle valve C and seat D preventing the needle valve from completely seating in its seat when desired grit flow is terminated. This results in grit particles of a size smaller than the selected size being allowed to continue to flow past the needle valve and seat when it is desired to terminate all grit flow to the nozzle. This problem is particularly noticeable when grit particles of an average grit size smaller than 60 grit are used in the system. The clogging of the needle valve results in erratical feeding of the normally smaller sized particles and other related problems.
There has not been a satisfactory means of consistently delivering a uniform supply of grit of a size from 60 to 250 grit until the emergence of the instant invention.